The present invention relates to a new and improved construction of a low-distortion audio amplifier circuit arrangement.
In its more particular aspects, the present invention relates to a new and improved construction of a low-distortion audio amplifier circuit arrangement comprising an amplifier, a source circuit containing a current-voltage source, and a transformer operatively connecting the source circuit to the amplifier.
Such amplifier circuit arrangements serve the purpose of amplifying audio signals with the least possible distortions and at the smallest possible distortion factor. Distortions of the audio signal occur in such amplifier arrangements both in the transformer as well as in the amplifier connected thereto. The distortions caused by the amplifier can be prevented in known manner by measures effective within and at the amplifier. The present invention is not concerned with preventing the distortion caused by the amplifier, but concerns itself with distortions which are caused by the transformer.
In an amplifier circuit arrangement as known, for example, from U.S. Pat. No. 3,828,269, granted Aug. 6, 1974, the magnetic operative control range of the transformer is maintained very small in order to eliminate to the greatest possible extent transformer-caused distortions. This is achieved by a negative or inverse feedback circuit interconnecting an output of the amplifier in the amplifier circuit arrangement and the transformer. Accordingly, the transformer is provided with negative or inverse feedback windings. The small magnetic operative control range of the transformer induces a small voltage in the winding associated with the source circuit and this, in turn, results in a very low input resistance of the amplifier circuit arrangement.
It is a disadvantage of such circuit arrangement that the input resistance thereof is too small for amplifying audio signals which originate from a signal source like, for example, a microphone. It would be conceivable to correctly match the output resistance of the microphone to the input resistance of the amplifier circuit arrangement by incorporating a suitable resistor in the source circuit. The input resistance could thus be increased to the desired degree. However, such matching, in turn, has the disadvantage that such resistor would generate noise which subsequently is further amplified by the amplifier and such is unacceptable in the case of audio signals.
In a further amplifier circuit arrangement as known, for example, from U.S. Pat. No. 3,656,831, granted Apr. 18, 1972, the source circuit is connected to an amplifier circuit via two transformers which are wound in a trifilar manner. The output of the amplifier circuit is again connected via a transformer. The amplifier circuit essentially comprises a main amplifier and an error amplifier. The arrangement aims at correcting distortions which are generated in the main amplifier. This amplifier circuit arrangement is predominantly used in the carrier frequency field which relates to the transmission of broad-band signals in lines or conductors.
Such amplifier circuit arrangement is unsuited for amplifying audio signals because the amplifier circuit arrangement has a low input impedance. The output impedance which must be low for processing audio signals, however, cannot be maintained sufficiently low in such amplifier circuit arrangement because the output circuit and one of the two transformers are connected in series in the source circuit. The input impedance and the output impedance of this amplifier circuit arrangement must be matched to the impedance of the related transmission line which means that the impedances are equal. Such equality of impedances cannot be used in audio signal amplifier circuit arrangements.
It is further known to prevent distortions generated by a transformer by designing the transformer to possess a large size and to be heavy in weight. However, such solution to the problem is undesirable in view of the trend towards miniaturizing electronic equipment.